Restart NFS on Gentoo host instead of starting

In the case that NFS is already started on a Gentoo host, /etc/init.d/nfs start will not reload exports, but fail. /etc/init.d/nfs reload is known to fail for some esoteric configurations of /etc/exports, restart should be safe for existing connections.
Merge pull request #2487 from hatsuseno/master

hosts/gentoo: Restart NFS on Gentoo host instead of starting [GH-2487]